Monday, 4 April 2011

MS Exchange error 9646 - MAPI session exceeded the maximum of 500 objects of type "objtFolder"

The error is logged in Windows application log on MS Exchange 2010 mailbox server :

Source: MSExchangeIS
Error ID: 9646
Description: Mapi session "xxxx: /o=OrganizationName/
exceeded the maximum of 500 objects of type "objtFolder".

At the user side, Outlook stops auto-refreshing and receiving
new email automatically,  but email is still delivered if
a user clicks on Send/Receive or Update Folder buttons.
Sending emails is not affected by this issue.

The issue occurs because Microsoft limitted the number
of server-side objects opened by a single MAPI session on
an Exchange server in order to prevent a single client
from exhausting resources on the server.

In order to find the exact number of folders and what
created those folders, run this cmdlet against the affected user's

Get-MailboxFolderStatistics user_email_address
Sort-Object ItemsInFolder -Descending |
ft Name, FolderPath > C:\countFolders.csv

Output to a .csv file will help to count the folders. Folder
path entry will help to determine to which parent folder the
listed subfolders belong and the name of the folder that
contains the most subfolders should help to figure out which
application or add-on (plug-in) created them. If the user in
question has several mailboxes attached to his/her Outlook,
then run the cmdlet against all those mailboxes and calculate the
total of all folders.

In my case, the issue was caused by an Outlook integrated
IP telephony application called IPFX for Outlook. It created
more then 150 folders per mailbox and it happened that some
users had 3 or more additional mailboxes attached to their
Outlook, which resulted in 600+ folders per MAPI session.

Microsoft suggests solving this issue by increasing the default
value for the object in question if the target number is not too
high. To increase the value, on Exchange server, go to:


and create a new key "MaxObjsPerMapiSession".
In the new key create a DWORD value "ObjtFolder".
Set a (decimal number) value for ObjtFolder to be
slightly above the number of the sum of all folders
accessed by the affected user(s). If this number is much higher
than the default value, then make sure that Exchange has sufficient
resources or remove the plug-in that caused the issue. The change
does not require a restart of the server or IS service.

It is advisable to monitor and log resource consumption
(RAM, page file, CPU, logical and physical disks) using
perfomance monitor for several weeks following the change
in order to observe the impact of the new settings.

More info:
Exchange Store Limits
Maximum open folder objects per logon has been changed
Your Exchange Server 2003 may stop responding after a MAPI client opens more than the default value of certain server objects


  1. Thank you! Great post, helped me a lot.

  2. Thanks a lot Zoran, helped me too!
    A sysadmin from Malta.